1. The Field of the Invention
The present invention relates generally to an apparatus and method for transforming chemical elements from a solid state to a gaseous state. More specifically, the present invention relates to an apparatus and method for vaporizing and cracking chemical elements for use in a deposition process such as during semiconductor device fabrication.
2. The Relevant Technology
Various chemical elements are used in conventional deposition processes performed during semiconductor device fabrication. For example, the chemical elements in Group V of the periodic table, such as phosphorus and arsenic, are commonly used as dopants in semiconductor processing technologies and are vital materials in several semiconductor devices. It is desirable to be able to convert the solid forms of these chemical elements into a form which may be subsequently combined with other chemical elements to create the desired product. To accomplish this, the chemical elements must first be vaporized.
Conventional techniques for converting chemical elements into vapor phase for semiconductor device fabrication employ a vacuum evaporation system. Generally, the vacuum evaporation system includes a heating unit and is in communication with a growth chamber for deposition of the element onto a substrate. The heating unit is used to supply the required energy to convert the element into a vaporous form, and the growth chamber is ideally operated under high vacuum conditions, as this ensures a high quality, non-contaminated crystal. One technique that employs such a vacuum evaporation system is molecular-beam epitaxy (MBE).
In MBE, a variety of sources can be employed for flux generation, and their design depends on the nature of source materials. The thermal effusion source or Knudsen-cells (k-cells) are used in nearly all MBE systems for deposition of semiconductor and/or dopant materials during semiconductor device fabrication. A k-cell includes a crucible containing a solid or liquid evaporant, which is radiatively heated by electrically insulated heater filaments wound around the crucible. A thermocouple, which is carefully positioned to ensure intimate contact with the crucible, registers the source material temperature and can, via a feed-back loop, control the power to the heater and thus the temperature of the source. Several layers of refractory metal foil (e.g., tantalum) are wrapped around the entire cell to minimize heat losses from the cell wall, with the major heat loss being from the effusion aperture.
Typically, the vaporization process yields varying ratios of chemical elements in monomeric, dimeric, and tetrameric forms. Conventional vaporization techniques for group V elements are unable to reduce the majority of the element to a monomeric or dimeric form, resulting in a substantial amount of tetrameric forms of the element. Such tetrameric forms of group V elements are undesirable from the standpoint of use in semiconductor device fabrication. The growth of a crystalline layer, which is required for semiconductor device applications, is best achieved when the monomeric (atomic) or the dimeric form of the element is used. Therefore, after vaporization of the chemical element, a method to efficiently convert clusters of tetrameric forms of the element into monomeric and dimeric forms is of substantial interest.
Several techniques for converting or xe2x80x9ccrackingxe2x80x9d tetrameric forms of chemical elements into monomeric or dimeric forms have been developed. Such techniques employ either extremely high temperatures or ultraviolet light to input the energy necessary to separate elemental clusters. Some of these systems, such as the ultraviolet light systems, are very mechanically complex with many small parts requiring continual adjustments in order to achieve optimal performance. For example, precise alignment of parts is necessary to focus an ultraviolet beam in a manner that will achieve efficient cracking of elemental clusters.
The disadvantages of cracking systems that employ extreme heat or ultraviolet light include the high maintenance and expense required to run and maintain such systems. The high expense is incurred through both the power consumption and the mechanical maintenance required. In addition, most known systems for evaporating and cracking chemical elements have separate evaporation and cracker cell units, which reduces the efficiency of providing the chemical elements in a desirable form for deposition.
It would therefore be of significant advantage to develop a simple, inexpensive, and efficient system which can perform both the functions of chemical element vaporization and cracking.
It is an object of the present invention to provide an apparatus for vaporizing and cracking chemical elements for use in a deposition process during semiconductor device fabrication.
It is another object of the present invention to provide such a vaporizing and cracking apparatus that is simple in design and manufacture, and inexpensive to use and maintain.
It is another object of the present invention to provide such a vaporizing and cracking apparatus which is a fully integrated or combined effusion system.
It is a further object of the present invention to provide such a vaporizing and cracking apparatus which is easy to use and operates at peak efficiency.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, an apparatus is provided for vaporizing and cracking chemical elements such as group V elements for use in a deposition process. The apparatus includes a vaporization cell integrally connected with a thermal cracker cell.
The vaporization cell has an inlet section in communication with a valve section defining a heating chamber capable of holding a chemical material to be evaporated or sublimated. A container such as a quartz boat is preferably disposed in the heating chamber for holding the chemical material. A heat source is positioned in the heating chamber and is capable of providing sufficient thermal energy to evaporate or sublimate the chemical material.
The thermal cracker cell is communicatively connected to an outlet of the vaporization cell, and includes a tapered elongated tube with a heating element such as a heating coil disposed therearound. The heating element is capable of providing sufficient thermal energy to dissociate molecular clusters of vaporized chemical material. This provides monomeric or dimeric chemical elements for use in a deposition process. The elongated tube is preferably composed of quartz and has a passageway with a diameter of a first dimension that narrows to a smaller second dimension toward an exit opening of the tube. The tube narrows in order to cause the gaseous clusters of elements to be separated so that the clusters can receive a greater amount of heat energy as a result of increased exposure to and decreased distance from the heating element. This exposure results in greater efficiency in separating elemental clusters, and allows the use of lower temperatures.
In a method for vaporizing and cracking a chemical material which utilizes the apparatus of the invention, a preselected amount of a chemical material is placed into the heating chamber, and the chemical material is heated to a first temperature sufficient to vaporize the chemical material. The temperature in the heating chamber can be monitored and adjusted for optimal vaporizing conditions. The vaporized chemical material is then directed to the elongated tube and is heated along the smaller second dimension of the passageway in the elongated tube to a higher second temperature sufficient to dissociate molecular clusters of vaporized chemical material. The dissociated chemical material can then be directed from the exit opening of the elongated tube to a vacuum chamber for deposition on a substrate.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.